Efficiency
July 12, 2021 7:32 PM   Subscribe

"Biking is the most efficient transportation." Be more specific, and/or cite your reliable source.

I first heard this like 20 years ago, a friend of mine did, too. Rather than spouting a factoid, I'd like to know what exactly I am talking about.
posted by aniola to Grab Bag (13 answers total) 8 users marked this as a favorite
 
Famously, Steve Jobs pulled on a graph that appeared in a 1973 issue of Scientific American for his "bicycle of the mind" quote. Looks like that graph came from Vol. 228, No. 3 (March 1973), pp. 81-91, which I, alas, do not have access to, but if you can get someone to share you a JSTOR login...
posted by straw at 7:42 PM on July 12, 2021 [4 favorites]


Here's an article that roughly does some calculations comparing bikes, walking, to a car.

Note bikes (generally) only carry one passenger, whereas cars carry multiple. So, it is not impossible for a high efficiency car (hybrid) to exceed bike efficiency.

For what it's worth, trains give bikes a run for their money due to the massive number of passengers that can be fit into a relatively small train.
posted by saeculorum at 7:45 PM on July 12, 2021 [1 favorite]


Apologies for not having a citation. What I have heard is that bicycles are one of the most efficient machines, in the sense of converting power into motion. They don't put off a lot of heat, and they don't they don't require a bunch of energy to power processes other than motion.

If you look at bicycling as transportation more holistically, that's a bit more complicated since you have to factor in the energy used to produce all the gear and replacement parts.
posted by Comet Bug at 7:47 PM on July 12, 2021 [9 favorites]


Ah, the Tucker graph, with its famous 0.15 cal / gram / km figure. Here's the SciAm article.

Accurate? Dunno. Not sure where my copy of Wilson has gone.
posted by scruss at 7:55 PM on July 12, 2021 [2 favorites]


In case it's of interest, it looks like the Tucker data was expanded upon and printed a few years later in American Scientist, 63, 413, which is pretty neat. By this very specific metric, bicycles have nothing on freight trains. (I'd never heard of any of this before, but ran into looking for the source for the Sci Am plot. There are earlier articles that are entirely about birds.)
posted by eotvos at 8:33 PM on July 12, 2021 [3 favorites]


Jstor link. I thought Jstor was supposed to be free.

Link to PDF
posted by mecran01 at 9:12 PM on July 12, 2021 [2 favorites]


Thanks for the PDF, mecran01.
posted by Bella Donna at 12:41 AM on July 13, 2021


Bike drivetrains are amazingly efficient. A top-quality bike is 98% efficient; even a cheap bike is probably in the high 80s (here's a slightly dated article).

However, humans as motors are not very efficient: only 20–25% of the calories in our food gets converted to useful work at the pedal.
posted by adamrice at 7:20 AM on July 13, 2021 [2 favorites]


I've always known the quote as "bikes are the most efficient form of human-powered transportation."
posted by rhizome at 6:09 PM on July 13, 2021


Response by poster: Thanks, everyone!
posted by aniola at 11:17 PM on July 13, 2021


Back of the envelope math - I have a non-Tesla electric car that gets about 4.0 miles per KWH, or 250 watt-hours per mile. Some random googling shows e-bikes taking about 15-20 watt-hours per mile. Picking a random value of 20 miles per hour from here is about 200 watts, so in the 3 minutes it takes to cover a mile it'll be about 10 watt-hours (200 watts * 1/20 hour).

So the car can take 4 people a mile at the cost of 65 watt-hours per person in 1 minute (at 60mph). The e-bike can take 1 person a mile at the cost of 20 watt-hours per person in 2 minutes (at 30mph). The cyclist can take 1 person a mile at the cost of 10 watt-hours per person in 3 minutes (at 20mph)

That's the best case for the car though, if there are fewer than 4 people it's much less efficient per person as mentioned above.
posted by true at 7:29 PM on July 14, 2021 [1 favorite]


Response by poster: Are these calculations accounting for the weight of the vehicle? I've heard that the weight of the vehicle shouldn't be greater than its load, so I would expect that to be accounted for in a calculation of efficiency. What happens when you do the back of the envelope calculation for watt-hours per mile per pound?
posted by aniola at 8:23 AM on July 15, 2021


I think if you wanted to do watt-hours per mile per pound the car it gets a lot closer. With 4 people in the car it weighs about 3750 lbs, so it's 250 / 3750= ~0.067 watt hours per pound mile. Assuming a combined weight of 200 lbs for the bike and rider it's 10 / 200 = ~0.05 watt hours per pound mile.

Using people instead of weight implicitly penalizes the car for being bigger; with the bike, most of those watt-hours (90% of them or so) go to moving the person, since the person is 90% of the weight of the bike/person combined system. With the car, the people are 20% of the weight of the combined car/person system.

So I was thinking of efficiency as "the overall energy output relative to the quantity of useful things moved the same distance" (where people are the useful things), but from a true efficiency perspective the car is probably better. We have solar panels (about 15% efficient) and the car itself is ~75% efficient at converting grid power to drive power. That means about 10% total efficiency.

Now we're even further on the back of the envelope, but it does seem to me that the car, fully loaded, would be more efficient at converting a given square footage of sunlight into either watt-hours per person-mile or watt-hours per pound-mile. Someone quoted above 20-25% human efficiency at converting calories to power, and wikipedia says that crop plants store about 0.5% of the sunlight in food. So that's about .01% "sun to power" efficiency, 100x worse than the car.

That's only in the steady state though, once you "pay off" the initial energy deficit that it took to make the car in the first place.
posted by true at 8:58 AM on July 15, 2021


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